Problem: What is $45\%$ of $20$ ?
Having $45\%$ of something means that you get $45$ out of every $100$ We can set up a proportion to find out what number is $45\%$ of $20$ $ \dfrac{{\text{percent}}}{100} = \dfrac{{\text{part}}}{{\text{whole}}}$ Which things do we know, and what are we trying to find? We know the ${\text{percent}}$ is $45$ . Is $20$ the ${\text{part}}$ or the ${\text{whole}}$ The $20$ is the ${\text{whole}}$ . We are trying to find the ${\text{part}}$ that makes up $45\%$ of it: $ \dfrac{{45}}{100} = \dfrac{{\text{part}}}{{20}}$ If we divide the denominator of the fraction on the left by $5$ , it will be the same denominator of the fraction on the right. To keep things equal, let's also divide the numerator on the left by $5$ $ \dfrac{{45} \div 5}{100 \div 5} = \dfrac{{\text{part}}}{{20}}$ $ \dfrac{{9}}{20} = \dfrac{{\text{part}}}{{20}}$ $ {9} = {\text{part}}$ So $9$ is $45\%$ of $20$.